1. Field of the Invention
This invention relates to clock generators for digital electronic circuits and, more particularly, to apparatus and a method for generating a clock signal which is tuned to a reference frequency.
2. Prior Art
The subject of clock generation for digital electronic circuits is covered in section 7.4 "Clock Generation" of a book by Carver Mead and Lynn Conway entitled Introduction to VLSI Systems, Addison-Wesley, October 1980, pages 233-236. Previously, the reference clock for operation of subsystems was obtained from a master clock signal, obtained from a crystal or other resonant circuit. For a self-contained VLSI system on an integrated circuit chip, it is often essential that clock signals be generated on the chip. The book recommends that, instead of dealing with electronic oscillator circuits, it is better to take a more basic approach beginning with an understanding of what clock signals are used for. A fundamental principle is stated, which is that the role of the clock in a synchronous system is to connect a sequence of desired events with time. The interval between clock transitions must be sufficient to permit the activities planned for that interval to occur. A clock signal then is more like a set of timers than an oscillator.
Timers can be built as a chain of inverter circuits. Clock generation circuits that use timers are elaborations on a ring oscillator circuit, which has an odd number of signal inversions around the ring. The ring oscillator has a period that is an odd sub-multiple of the delay time twice around the ring. To avoid having the ring oscillate at a harmonic, clock signals are suppressed during initialization by using a gate in series with the ring to inhibit operation until an initializing signal is provided.